This invention relates to the control of an internal combustion engine in response to combustion pressure and particularly to the control of engine dilution by EGR, variable valve lift or similar apparatus. The development of piezoelectric pressure sensors suitable for combustion pressure sensing and microcomputers suitable for internal engine control has created a great deal of interest in engine control in response to combustion pressure, since it is recognized that a great deal of useful information about the combustion process is derivable from the combustion pressure curve.
One approach to the analysis of a combustion pressure curve is the examination of its harmonics. An approximation of a typical combustion pressure waveform and its harmonics at the engine firing frequency, twice this frequency and four times this frequency are shown in FIGS. 5b, 5c, 5d and 5e, respectively. I have discovered certain relationships between these and other harmonics that show promise in engine control. One such relationship involves the amplitude ratio of corresponding peaks of two different harmonics of the combustion pressure curve, particularly of the first harmonic (fundamental) at the engine firing frequency and another harmonic or subharmonic. When a cylinder or cylinders begin to misfire, the ratio of the other harmonic or subharmonic to the fundamental increases. By establishing a reference ratio and comparing the actual ratio thereto, the dilution of the combustible charge may be varied to decrease the misfiring as desired in closed loop control.